Resin lens array and optical writing head

ABSTRACT

Cone-shaped projections are provided outside a lens forming area in the middle on one surface of a resin lens plate and depressions to be fitted to these projections are formed on the other surface. A resin lens array is formed by stacking the resin lens plates one over another through fitting these projections and depressions to each other. A fitting depression having a flat supporting seat face at a position on an optical path of light emitted by light-emitting elements of a light-emitting element array chip is formed in the lens holder, and the resin lens array is placed in the fitting depression. A transparent cover composed of a flat and smooth plate is arranged over the output surface of the resin lens array and a metal retainer is attached to the transparent cover so that a pressing pressure is always applied to the resin lens array.

TECHNICAL FIELD

The present invention relates to an optical write head mounted on anelectro-photographic printer, and collecting and projecting lightoutputted from a light-emitting element array through a resin lens arrayon a photosensitive member.

BACKGROUND ART

An electro-photographic printer performs printing by forming a latentimage on a photosensitive drum through exposure of the drum, developingthis latent image with toner, transferring the toner onto a sheet ofpaper and fixing the toner on the paper by means of heat or the like.

An exposure process for forming a latent image is classified into an LEDoptical system and a laser optical system. In a write head (hereinafter,referred to as an optical write head) of an LED optical system, lightoutputted from the LED is irradiated onto a photosensitive drum throughan erecting unit magnification rod lens array.

FIG. 1 is a sectional view taken perpendicularly to the longitudinaldirection of an optical write head to be mounted on a conventionalelectro-photographic printer. In this optical write head, a plurality oflight-emitting element array chips 64 each having light-emitting elementarranged in line are mounted in the scanning direction on a substrate 63and an erecting unit magnification rod lens array 61 which is longer inthe scanning direction is fixed at a position on an optical path oflight outputted by light-emitting elements of the light-emitting elementarray 64 by a housing 62 made of resin. And the outer edge portionsperpendicular to the longitudinal direction of the substrate 63 areengaged with the leg end portions of the housing 62. In addition, a heatsink 60 for radiating heat of the light-emitting element array chips 64is provided under the substrate 63, and the housing 62 and the heat sink60 are fixed by metal retainers 66 with the substrate 63 between them.

A photosensitive drum 65 is provided above the rod lens array 61. Therod lens array 61 forms a latent image on the photosensitive drum 65 bycollecting light of light-emitting elements of the light-emittingelement array chips 64 and exposing the photosensitive drum.

An erecting unit magnification imaging optical system can be verycompact by using an optical write head as described above. Since a rodlens array is manufactured by arranging and fixing a number of rod lenselements with resin, however, some irregularity or the like inarrangement of rod lens elements is liable to occur. Such irregularityin arrangement has an influence on the resolution of a rod lens arrayand makes a cause of image irregularity and the like in a recent machineof high resolution (resolution of 1,200 dpi for example).

In order to suppress the occurrence of such an arrangement irregularity,it is conceived to substitute a resin lens array for a rod lens array asdescribed above. A resin lens array, which realizes an erecting unitmagnification imaging optical system by stacking one over another two ormore flat lens array plates each having a number of single lenses formedon a transparent substrate, has no possibility of making any arrangementirregularity in the lens array thanks to manufacturing a lens arrayplate having a number of single lens formed in it by injecting resininto a mold.

An optical write head used in an electro-photographic printer mountedwith such a resin lens array is disclosed in Japanese Patent Laid-OpenPublication No.2000-221, 445. The optical write head disclosed in thisofficial gazette is a head of an erecting unit magnification opticalsystem improving the resolution by stacking one over another a pluralityof resin-molded lens array plates.

However, an optical write head disclosed in Japanese Patent Laid-OpenPublication No.2000-221, 445 has the following problems.

In case of stacking a plurality of lens array plates one over another,it is necessary to align them so that the opposing vertexes of lensesare made small in deviation of position relative to one another thereof.In order to use a resin lens array in a high-resolution optical writehead, it is necessary to make the size of a single lens as small aspossible, and for example in a resin lens array to be used in an opticalwrite head having a resolution of 1200 dpi, a single lens needs to be0.4 mm or less in diameter according to simulation and naturally thealignment of lenses must be adjusted accurately in the order ofmicrometers. Therefore, a high accuracy is required for the alignment oflenses and in a structure where a plurality of lens array plates arestacked on one another, the alignment of them is very difficult.

And since a developing roller which discharges toner is located near theoutput surface of a resin lens array, foreign matters such as toner andthe like are floating there and the foreign matters stuck to the outputsurface of the resin lens array cause deterioration in performance ofthe optical write head, and therefore removal of the stuck foreignmatters is periodically performed by a method of wiping off with softcloth or the like. However, since a resin lens array using convex lenseshas a surface which is depressed and protruded in shape, it is difficultto clean. And since resin generally used in an optical lens is socomparatively soft as to be B to HB (JIS K5401) in pencil hardness, thesurface of a resin lens array is liable to be scratched when stuckmatters are removed and there is the possibility of deteriorating theoptical performance of it.

And resin used generally in an optical lens is 90 to 93% intransmittance (ASTM D1003) and since a lens array which is as bright aspossible is needed for high-speed printing, a resin lens array isdesired to be made as thin as possible but thinning it leads toremarkable degradation of a shape-retaining ability of the resin lensarray itself. Mounting a resin lens array having a poor shape-retainingability on a head with a high positioning accuracy leads to morecomplication and higher precision in structure of members supporting theresin lens array and results in increasing the cost of components.

And since resin used in an optical lens is generally high in coefficientof thermal expansion, in case of sticking and fixing a resin lens arrayto a housing formed out of a rigid material, the occurrence of straincaused by the difference in coefficient of thermal expansion between theresin lens array and the housing deforms the resin lens array andthereby lowers the accuracy of position of the resin lens array. Andsince in a resin lens array an adhesive agent is generally used forfixing lens array plates to each other, the slippage in stackingposition caused by exfoliation in the adhesive interface between thelens array plates deteriorates remarkably the quality of image.

And since a resin lens array is low in rigidity and has a poorself-shape-retaining ability, to fix the resin lens array highly flat,it needs to be fixed in position along a datum plane of another memberwhich is high in flatness. In this case, for example a method ofattaching a resin lens array to a datum plane of a housing and fixingthe housing and the resin lens array to each other with an adhesiveagent is conceivable, but this method needs to keep the whole resin lensarray attached to the housing pressed into the housing side until theadhesive agent is hardened so as to be fixed along the datum plane andrequires a considerably long time in the manufacturing process.

And when a light beam reaches the interface between transparent mediawhich is different in refractive index from each other, a part of thelight beam is reflected by the interface and the other parts passthrough the interface and enters the next medium. The quantity ofreflected light at an incident angle of 0° between an optical resin lensand an air layer is about 4%. That is to say, the transmittance in casethat a light beam passes through a single lens array plate is:(1−0.04×2)×100=0.92×100=92 (%)In an optical system in which n lens array plates are stacked one overanother, since a light beam passes through the n lens array plates, thetotal transmittance becomes (0.92^(n)×100) %. This means using aplurality of lens array plates reduces the total quantity of transmittedlight. For example, the total transmittance of a structure in which lensarray plates of three layers and a protective cover of one layer areused is 0.92^(n)×100=72% and results in bringing about the reduction inquantity of light of 28%.

DISCLOSURE OF THE INVENTION

The present invention has been performed paying attention to suchconventional problems, and an object of the invention is to provide anoptical write head mounted with a resin lens array enabling low-priceand high-quality printing.

A resin lens array according to a first aspect of the present inventionis formed by stacking one over another a plurality of resin lens plateseach having spherical or aspherical microlenses regularly arranged atspecific intervals on a flat plate. The resin lens plate has cone-shapedprojections formed at specific intervals outside the area in whichlenses are formed on one surface and depressions to be fitted to theseprojections formed outside the area in which lenses are formed on theother surface, and the resin lens array is formed by stacking the resinlens plates one over another through fitting the projections and thedepressions to each other.

Or the resin lens plate has projections which is triangle-shaped insection and continuous or which is triangle-shaped in section andsuccessive at specific intervals outside the area in which lenses areformed on one surface and depressions to be fitted to these projectionsformed outside the area in which lenses are formed on the other surface,and the resin lens array is formed by stacking the resin lens plates oneover another through fitting the projections and the depressions to eachother.

And an optical write head according to a second aspect of the presentinvention is a head which collects and projects light outputted from anlight-emitting element array chip having light-emitting elementsarranged in line on a photosensitive member through a resin lens arrayformed by stacking one over another a plurality of resin lens plateseach having spherical or aspherical microlenses regularly arranged atspecific intervals on a flat plate, and the resin lens plate hasprojections outside the area in which lenses are formed on one surfaceand depressions to be fitted to these projections formed outside thearea in which lenses are formed on the other surface, and the resin lensarray is formed by stacking the resin lens plates one over anotherthrough fitting the projections and the depressions to each other.

Furthermore, an optical write head according to a third aspect of thepresent invention is a head which collects and projects light outputtedfrom an light-emitting element array chip having light-emitting elementsarranged in line on a photosensitive member through a resin lens arrayformed by stacking one over another a plurality of resin lens plateseach having spherical or aspherical microlenses regularly arranged atspecific intervals on a flat plate, and the resin lens array is housedin an opening formed in a supporting means for supporting the resin lensarray and is supported by the supporting means through engaging portionshaving projections at the opening side.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view taken perpendicularly to the longitudinaldirection of an optical write head to be mounted on a conventionalelectro-photographic printer.

FIG. 2A is a plan view of a resin lens array according to a firstembodiment.

FIG. 2B is a side view of the resin lens array according to the firstembodiment.

FIG. 3 is a magnified side view of the resin lens array.

FIG. 4A is a plan view showing another example of a resin lens array.

FIG. 4B is a side view showing said another example of the resin lensarray.

FIG. 5 is a diagram showing the conditions of film type, film thicknessand number of layers in case of simulating the transmittance dependingon an AR-coated film.

FIG. 6 is a diagram showing the relation between transmittance andwavelength in case of forming a two-layer AR coat film.

FIG. 7 is a diagram showing the relation between transmittance andwavelength in case of forming a three-layer AR coat film.

FIG. 8 is a diagram showing the relation between transmittance andwavelength in case of forming a four-layer AR coat film.

FIG. 9 is a diagram showing the relation between transmittance andwavelength in case of forming a five-layer AR coat film.

FIG. 10A is a plan view of an optical write head according to a secondembodiment.

FIG. 10B is a front view, partly in section, of an optical write headaccording to the second embodiment.

FIG. 10C is a sectional view taken perpendicularly to the longitudinaldirection of an optical write head according to the second embodiment.

FIG. 11 is a magnified sectional view of an optical write head.

FIG. 12 is a perspective view of a heat sink and a substrate.

FIG. 13 is a magnified sectional view showing a variant example of anoptical write head.

FIG. 14A is a sectional view of the middle part of an optical write headaccording to a third embodiment taken perpendicularly to thelongitudinal direction of the head.

FIG. 14B is a partial front view of the optical write head according tothe third embodiment.

FIG. 15 is a perspective view of a lens holder and a resin lens array.

FIG. 16 is a sectional view of an end portion in the longitudinaldirection of the lens holder and the resin lens array taken along to thelongitudinal direction of them.

FIG. 17 is a sectional view taken along line B-B′ of FIG. 14.

FIG. 18 is a diagram showing an equivalent circuit of a self-scanninglight-emitting element array.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, a first embodiment of the present invention is described withreference to the drawings.

A resin lens array according to the first embodiment of the presentinvention is formed by closely stacking resin lens plates each having anumber of lenses formed on one or both faces of itself one over anotherso as to have at least three surfaces having lenses formed. FIG. 2A is aplan view of a resin lens array according to the first embodiment to bemounted on an optical write head, FIG. 2B is a side view of the resinlens array and FIG. 3 is a magnified side view of the resin lens array.

A resin lens array 10 is composed of three resin lens plates 12. Theresin lens plate 12 is in the shape of a narrow and long rectangle, andhas a number of convex microlenses arranged in a lens forming area 18 ofthe middle part of it. The microlenses are formed on both faces of theresin lens plate 12. A convex microlens formed in the resin lens plate12 may be either a spherical lens or an aspherical lens.

And the resin lens plate 12 has cone-shaped projections 14 for aligninga resin lens plate 12 to be stacked which are provided outside a lensforming area 18 on one surface of the resin lens plate 12, and hasdepressions 16 to be fitted to these cone-shaped projections 14 providedon the other surface. The resin lens array 10 is formed by stacking theresin lens plates 12 one over another through fitting the projections 14into the depressions 16.

It is desirable that the projections 14 and the depressions 16 foralignment are arranged at intervals of 15 to 60 mm in the longitudinaldirection around the lens forming area 18 of the resin lens plate 12, asshown in FIGS. 2A and 2B.

These projections and depressions to be provided on a resin plate areformed by injection-molding resin. An injection molding method injectsresin into a metal mold having the upper and lower metal molds closed bymeans of an extruder and then hardens the resin inside the metal mold bygradually cooling it, and thereafter opens the upper and lower metalmolds and pulls out a resin lens plate. When a side face havingprojections is provided along the direction of pulling out from themetal mold, this side face makes a mold-releasing resistance between themetal mold and the resin lens plate to provide a one-sided load in amold-releasing operation and thus there is the possibility of deformingthe resin lens plate. Therefore, it is desirable that the projection anddepression each are in the shape of an easy-released taper having agradient of 3° or more.

FIG. 4A is a plan view showing another example of a resin lens array,and FIG. 4B is a side view of the resin lens array. A continuousprojection 14 a, which is triangular in section, and a continuousdepression 16 a to be fitted to this projection are formed around a lensforming area 18 a of a resin lens plate 12 a to form a resin lens array.

Parts for aligning resin lens plates may be in the shape of a projectionand a depression to be fitted to this projection which is triangular insection and continuous or may be in the shape of projections anddepressions to be fitted to these projections which are triangular insection and successive at regular intervals.

And an AR (anti-reflection) coat film (antireflection coating) is formedon each of both surfaces of a resin lens plate to form a resin lensarray described above. It is desirable to form an AR coat film on eachof both surfaces of a resin lens plate in order to reduce reflectedlight. The AR coat film is formed by a vapor deposition method, asputtering method or the like. This embodiment has formed an AR coatfilm by stacking an Al₂O₃ film (121 nm in thickness) and a SiO₂ film(134 nm in thickness) in order on each of both surfaces of a resin lensplate by means of a vapor deposition method. The transmittance of oneresin lens plate at a wavelength of 750 nm which is a general emittedlight wavelength of a light-emitting element has been 92% in case ofhaving no AR coat film but has been improved to 97.5% by the AR coatfilm. In case of stacking n resin lens plates one over another, thetotal transmittance has been able to be improved from 0.92^(n)×100% to0.975^(n)×100%.

The composition of an AR coat film is not limited to two layers of anAl₂O₃ film (121 nm in thickness) and a SiO₂ film (134 nm in thickness)but may be different in film thickness or different in number of layersand may be different in type of films in order to enhance the opticaltransmittance. The transmittance can be more improved depending on thefilm type, the film thickness, the number of layers and the like of anAR coat film.

FIG. 5 is a diagram showing the conditions of film type, film thicknessand number of layers in case of simulating the transmittance dependingon an AR coat film. FIGS. 6 to 9 show the relation between transmittanceand wavelength in case of forming an AR coat film under the conditionsshown in FIG. 5 taking the film type, film thickness and number oflayers as parameters.

FIG. 6 is a diagram showing the relation between transmittance andwavelength in case of forming an AR coat film by stacking two layers ofan Al₂O₃ film and a SiO₂ film in order on each of both surfaces of aresin lens plate, FIG. 7 is a diagram showing the relation betweentransmittance and wavelength in case of forming an AR coat film bystacking three layers of an Al₂O₃ film, a TiO₂ (or Ta₂O₅) film and aSiO₂ film in order on each of both surfaces of a resin lens plate, FIG.8 is a diagram showing the relation between transmittance and wavelengthin case of forming an AR coat film by stacking four layers of a TiO₂ (orTa₂O₅) film, a SiO₂ film, a TiO₂ (or Ta₂O₅) film and a SiO₂ film inorder on each of both surfaces of a resin lens plate, and FIG. 9 is adiagram showing the relation between transmittance and wavelength incase of forming an AR coat film by stacking five layers of a SiO₂ film,a TiO₂ (or Ta₂O₅) film, a SiO₂ film, a TiO₂ (or Ta₂O₅) film and a SiO₂film in order on each of both surfaces of a resin lens plate.

For example, by using a four-layer AR coat film of FIG. 5 having a TiO₂film, a SiO₂ film, a TiO₂ film and a SiO₂ film stacked one over anotherin order on each of both surfaces of a resin lens plate, it is possibleto improve the transmittance of a resin lens plate to 99% or more overthe range of 620 nm to 1 μm or more in wavelength.

In case of stacking and fixing resin lens plates to one another, themethod applies a UV (ultra-violet) hardening adhesive agent todepressions for alignment and then fits projections for alignment intothese depressions, irradiates the projections and depressions foralignment and the vicinity of them with ultraviolet rays, hardens theadhesive agent and thereby aligns and fixes the resin lens plates to oneanother.

At this time, as shown in FIG. 3, a hole 20 for making an excessiveadhesive agent escape when fitting a projection 14 and a depression 16of resin lens plates to each other may be formed at the bottom of thedepression 16. By forming such a hole 20, it is possible to suppressoverflow of the adhesive agent from the depression 16. In case that adepression is continuously formed as shown in FIG. 4A and 4B, a groove(not illustrated) for making an adhesive agent escape is formed in thebottom of the depression 16 a.

In case of stacking a plurality of resin lens plates as described above,the alignment of each lens can be performed with high accuracy by simpleoperations through fitting the depressions and projections to eachother.

Next, a second embodiment of the present invention is described withreference to the drawings.

FIG. 10A is a plan view of an optical write head according to a secondembodiment, FIG. 10B is a front view, partly in section, of the opticalwrite head, FIG. 10C is a sectional view taken perpendicularly to thelongitudinal direction (scanning direction) of the optical write head,and FIG. 11 is a magnified sectional view of the optical write head.

A substrate 24 is fixed on a heat sink 28 and a plurality oflight-emitting element array chips 26 each having light-emittingelements arranged in line are mounted along the scanning direction onthe substrate 24. Parts of the bottom of legs of a lens holder(supporting means) 30 are engaged with the upper faces of end portionstaken perpendicularly to the scanning direction of chips of thesubstrate 24. And projections provided on the end portions of the legsof the lens holder 30 are fixed by an adhesive agent to the side facesof the heat sink 28 as providing gaps 38 between the heat sink 28 andthe lens holder 30.

The lens holder 30 has a fitting depression having a flat supportingseat face formed at a position on an optical path of light emitted bylight-emitting elements of a light-emitting element array chip 26, and aresin lens array 10 according to the first embodiment is placed in thefitting depression of the lens holder 30. A transparent cover 34composed of a flat plate is arranged over the output surface of theresin lens array 10 and metal retainers 36 are attached to thetransparent cover 34 so that a pressing pressure is always applied tothe resin lens array 10.

The heat sink 28 is made of aluminum and preferably uses a materialcapable of being made by a drawing process in order to reduce the cost.Nonferrous metal or metal material other than aluminum may be used as amaterial for the heat sink 28.

FIG. 12 is a perspective view of a heat sink and a substrate. The heatsink 28 is provided with a plurality of pins 40 for performing alignmentand the substrate 24 to be mounted with light emitting array chips isalso provided with holes 42 for having the pins 40 inserted into them atpositions corresponding to the pins 40. The substrate 24 and the heatsink 28 are aligned with each other by inserting the pins 40 into theholes 42.

As shown in FIG. 12, the holes 42 provided at both end portions in thelongitudinal direction of the substrate 24 may be slots which is longerin the scanning direction of chips. By making the holes at both endportions slot-shaped, it is possible to absorb the strain caused by thedifference in coefficient of thermal expansion between the substrate 24and the heat sink 28 through the holes 42.

A material for the substrate 24 is preferably aluminum which is the samematerial as the heat sink 28 but may be any of a glass-epoxy substrate,a metal substrate and a nonferrous metal substrate.

A resin material capable of being injection-molded is preferably usedfor the lens holder 30. Since it is possible to absorb the strain causedby the difference in coefficient of thermal expansion between thesubstrate 24 and the heat sink 28 by using a material having acoefficient of thermal expansion approximate to that of the heat sink 28for the lens holder 30, this embodiment uses a resin materialcomparatively low in coefficient of thermal expansion having carbonfiber mixed with the resin. A resin material having glass short fibermixed with it may be used for the lens holder 30.

As shown in FIG. 11, the lens holder 30 and the heat sink 28 are fixedtogether by providing a gap 38 between the heat sink 28 and the lensholder 30 and fixing projections provided at the leg end portions of thelens holder 30 to the side faces of the heat sink 28 by means of anadhesive agent 32.

The reason for the gap 38 being provided between the heat sink 28 andthe lens holder 30 is to allow the lens holder 30 to be fixed on theheat sink 28 under the condition of the lens holder 30 contacting withnot the surface of the heat sink 28 on which the substrate 24 ismounted, but upper surface of the substrate 24 in order to secure withhigh accuracy an working distance between the light emitting points of alight-emitting element array chip 26 mounted on the substrate 24 and theincident surface of a resin lens array 10 supported by the lens holder30.

A resin lens array 10 composed of a plurality of resin lens plates isplaced in a fitting depression of the lens holder 30. It is necessarythat the fitting depression of the lens holder 30 having the resin lensarray 10 placed in it is made rather longer in shape than the resin lensarray 10 in the longitudinal direction in consideration of thedifference in coefficient of thermal expansion between the materials.

For example, on the assumption that the coefficient of thermal expansionof a lens holder is 2.2×10⁻⁵/° C., the coefficient of thermal expansionof a resin lens array is 8.0×10⁻⁵/° C., a temperature change is 40° C.and the exposure width (length of the resin lens array) of an opticalwrite head of A3-sized paper in Japanese Industrial Standard is about320 mm, the following conditions are obtained:

-   -   Exposure length=320 mm,    -   Temperature difference=40° C.,    -   Difference in coefficient of thermal expansion=(8.0−2.2)×10⁻⁵/°        C.=5.5×10⁻⁵/° C.,

Difference in displacement between the members=320×40×5.8×10⁻⁵=0.74 mm,

-   -   and the difference in displacement caused by the difference in        coefficient of thermal expansion between the lens holder and the        resin lens array becomes 0.74 mm. In order to absorb this        difference in displacement caused by the difference in        coefficient of thermal expansion, the fitting depression to have        the resin lens array placed in it needs to be made 0.8 mm or        longer in shape than the resin lens array.

And the resin lens array 10 and the lens holder 30 may be aligned witheach other by providing pins for alignment both end portions in thelongitudinal direction of a flat supporting seat face of the fittingdepression, providing holes to have the pins inserted into them atpositions corresponding to these pins in the longitudinal direction ofthe resin lens array 10 and inserting the pins into these holes. In thiscase, when one of the holes to have the pins for alignment inserted intothem is made into a round hole and the other is made into a slot longerin the chip scanning direction, in case that a difference indisplacement caused by the difference in coefficient of thermalexpansion occurs between the lens holder 30 and the resin lens array 10,the slot of the resin lens array 10 can absorb this difference indisplacement.

After the resin lens array 10 has been placed in the fitting depressionof the lens holder 30, a transparent cover 34 is attached over it andmetal retainers 36 are attached to the transparent cover 34. The metalretainer 36 is formed out of a plate spring, and one end portion of theplate spring is inserted into a groove provided in a side face of thelens holder 30 and the other end portion is pressed against theperipheral part of the surface of the transparent cover 34 correspondingto the position outside the lens forming area of the resin lens array10.

Since a structure in which the resin lens array 10 is pressed downagainst the lens holder 30 by the metal retainers 36 such as platesprings and the like is made, the stress between both the components canbe released. The transparent cover 34 and the resin lens array 10 arefixed to the lens holder 30 by these metal retainers 36.

In order to prevent dust such as toner and the like from coming into ahead, it is necessary to apply a sealant such as silicone or the like tothe joint of the resin lens array 10 and the lens holder 30, and inorder to release the stress between the above-mentioned components thesealant is preferably about 100 (JIS-A) in hardness.

It is enough that the transparent cover 34 is made of a transparentmaterial, and this embodiment has used a soda-lime glass plate. Thetransparent cover 34 may also be provided with an AR coat film.

If a glass plate the surface of which is flat and smooth is adopted asthe transparent cover 34, since the surface of it can be kept hard, evenwhen toner is stuck, the dirt can be easily removed with waste cloth andthe like and the resin lens array is difficult to be scratched andtherefore the deterioration in performance is difficult to occur.

FIG. 13 is a magnified sectional view showing a variant example of anoptical write head shown in FIG. 11. The optical write head shown inFIG. 13 covers the outer circumference of a heat sink 28 with a metalretainer 37 such as a plate spring which is extended to have enoughlength to cover the outer circumference of a heat sink 28, presses theheat sink 28 against a lens holder 31 by means of this metal retainer 37and thereby fixes the lens holder 31 and the heat sink 28 to each otherin this state. Since the other points are the same as FIG. 11, thedescription of them is omitted.

In the optical write head shown in FIG. 13, the lens holder 31 and theheat sink 28 can be fixed to each other without using an adhesive agent.

Next, a third embodiment of the present invention is described withreference to the drawings.

FIG. 14A is a sectional view of the middle part in the longitudinaldirection (scanning direction) of an optical write head according to athird embodiment taken perpendicularly to the longitudinal direction ofthe head, and FIG. 14B is a partial front view of the optical writehead.

A plurality of light-emitting element array chips 44 each havinglight-emitting elements arranged in line are mounted on a substrate 43.A resin lens array 10 according to the first embodiment is supported byengaging portions formed on the lens holder. (supporting means) 45 in alens holder 45 and at a position on an optical path of light emittedfrom light-emitting elements of these light-emitting element array chips44. The resin lens array 10 is formed out of three resin lens plateseach having convex microlenses arranged regularly at specific intervalson a transparent flat plate stacked one over another.

And the outer edges taken perpendicularly to the longitudinal directionof the substrate 43 are engaged with the leg end portions of the lensholder 45. And a heat sink 46 for discharging heat of the light-emittingelement array chips 44 is provided under the substrate 43, and the lensholder 45 and the heat sink 46 are fixed to each other by metalretainers 47 with the substrate 43 between them.

An FPC (flexible printed circuit: flexible substrate) 48 for taking inan electric signal from a gap provided between the lens holder 45 andthe heat sink 46 is connected to the substrate 43. And a datum pin 49 tobe used as a datum point for alignment of a photosensitive drum 50 atthe time of assembling an optical write head in an electro-photographicprinter is provided at an end portion in the longitudinal direction ofthe lens holder 45. This photosensitive drum 50 is provided above theresin lens array 10.

FIG. 15 is a perspective view of the lens holder and the resin lensarray shown in FIG. 14. As shown in FIG. 15, an opening 51 having ashape capable of housing the resin lens array 10 is formed in the middleof the lens holder 45, and engaging portions 52 each having a projectionat the opening 51 side are provided at regular intervals over the wholerange of the longitudinal direction side of the opening 51 and theperpendicular direction side to the longitudinal direction of theopening 51 around the opening 51.

The engaging portions 52 are molded out of resin into one body togetherwith the lens holder 45. The resin lens array 10 is supported in thelens holder 45 by inserting the resin lens array 10 into the opening 51from above and pressing the end portions of the resin lens array 10against the projections of the engaging portions 52.

In the above-mentioned embodiment, the engaging portions 52 are providedat both of the longitudinal direction sides and the perpendiculardirection sides to the longitudinal direction of the opening 51 aroundthe opening 51, but may be provided at only the longitudinal directionsides or only the perpendicular direction sides to the longitudinaldirection. And the engaging portions may be provided continuously orsuccessively at regular intervals at the longitudinal direction sidesand/or the perpendicular direction sides to the longitudinal direction.Further, they may be provided not only at regular intervals but also atirregular intervals.

In an optical write head of the present invention, since a lens holderhas engaging portions for engaging a resin lens array around an openingas described above, the resin lens array can be easily mounted only bylightly pressing down the resin lens array into the lens holder.Therefore, it is possible to greatly shorten a time for assembling theresin lens array in the lens holder and reduce the cost of production.

FIG. 16 is a sectional view of an end portion in the longitudinaldirection of the lens holder and the resin lens array taken along thelongitudinal direction of them. FIG. 16 shows a state when an opticalwrite head is subjected to a temperature change.

When a lens holder 45 provided with the above-mentioned engagingportions 52, said lens holder mounted with a resin lens array 10, issubjected to a temperature change, the difference in length caused bythe difference in coefficient of thermal expansion between the lensholder 45 and the resin lens array 10 occurs, but the engaging portions52 formed out of a material capable of absorbing the difference indisplacement caused by the difference in coefficient of thermalexpansion can absorb the difference in displacement thanks to a factthat the engaging portions 52 change in shape as shown in dashed linesof FIG. 16. At this time, since the distance LO between thelight-emitting element array chips 44 mounted on the substrate 43 andthe resin lens array 10 is not changed, the deterioration in printingquality can be prevented.

In an optical write head of the present invention, since a resin lensarray is not fixed to a datum plane of a lens holder, even if thedifference in displacement caused by the difference in coefficient ofthermal expansion occurs between the lens holder and the resin lensarray due to a temperature change, it is possible to absorb thedifference in displacement with the variation in position of theengaging portions and prevent the lens holder and the resin lens arrayfrom warping or the resin lens array from being broken.

And since a load F pressing down the resin lens array to the lens holderside (toward the light-emitting elements) is applied to the resin lensarray by inclined portions 52 a provided on the engaging portions 52,there is no variation in position in the focal distance directionbetween the light-emitting elements and the lens array and thus theoptical performance is not changed. Therefore, the deterioration inprinting quality can be prevented.

It is preferable to make the inclined portion 52 a into an inclined faceas shown in FIG. 16. And when the engaging portion 52 is provided so asto extend long in the direction of a normal line to the resin lens array10, since the distance from the fulcrum to the point of action islengthened, a load to displacement of the engaging portion 52 can bemade small.

FIG. 17 is a sectional view taken along line B-B′ of FIG. 14. FIG. 17shows a state of applying a sealing material 53 between the resin lensarray 10 and the lens holder 45 in order to prevent the deterioration inperformance due to invasion of foreign matter such as toner or the likeinto a head.

In a place where no engaging portion 52 is provided, a gap appearsbetween the resin lens array 10 and the lens holder 45, but the invasionof foreign matter such as toner or the like into the head can beprevented by applying a sealing material 53 such as caulking silicone orthe like thereto.

Since the resin lens array 10 is fixed to the lens holder 45 by theengaging portions 52, the sealing material 53 can be applied withoutnecessity of a jig or the like for fixing the resin lens array 10 to thelens holder 45 and therefore the operability is improved.

In an optical write head of the present invention, since engagingportions are provided on the area of a lens holder to be mounted with aresin lens array, it is possible to easily fix the resin lens array tothe lens holder and fix the resin lens array to the lens holder withoutnecessity of a jig. Further, also when applying a sealing material to anend portion of the resin lens array, since it is not necessary to fixboth of them together by means of a jig, a head can be easilymanufactured.

In the above-mentioned embodiments, a lens holder formed out of resinhas been described, but without limiting to resin, metal materials andthe like which are capable of absorbing the difference in displacementcaused by the difference in coefficient of thermal expansion can be alsoused. And the other parts than engaging portions may be formed out ofresin and only the engaging portions may be formed out of metal orsprings.

And in the above-mentioned embodiments, a self-scanning light-emittingelement array chip can be used as a light-emitting element array chip. Aself-scanning light-emitting element array chip is a light emittingarray chip having a self-scanning circuit built in it and a function oftransferring a light emitting point in order.

With regard to a self-scanning light-emitting element array, JapanesePatent Laid-Open Publication No.Hei 1-238, 962, Japanese PatentLaid-Open Publication No.Hei 2-14, 584, Japanese Patent Laid-OpenPublication No.Hei 2-92, 650, Japanese Patent Laid-Open PublicationNo.Hei 2-92, 651 and the like have disclosed that it makes a mountingprocess simple as a light source for a printer head, that it can makesmall the interval between light-emitting elements, that it enables acompact printing head to be manufactured, and the like. And JapanesePatent Laid-Open Publication No.Hei 2-263, 668 has proposed aself-scanning light-emitting element array having a structure in which atransferring element array is separated as a shift part from alight-emitting element array which is a light emitting part.

FIG. 18 shows an equivalent circuit of a self-scanning light-emittingelement array having a structure in which a shift part and a lightemitting part are separated from each other. The shift part has transferelements T₁, T₂, T₃ and so forth, and the light emitting part has writelight-emitting elements L₁, L₂, L₃ and so forth. The transfer elementand light-emitting element each are composed of a three-terminal lightemitting thyristor. The shift part uses diodes D₁, D₂, D₃ and so forthin order to connect the gates of transfer elements to each otherelectrically. V_(GK) is a power source (ordinarily 5 volts) and isconnected through load resistors R_(L) to gate electrodes G₁, G₂, G₃ andso forth of the respective transfer elements. And the gate electrodesG₁, G₂, G₃ and so forth of the transfer elements are connected also tothe gate electrodes of the write light-emitting elements. A start pulseφ_(S) is applied to the gate electrode of the transfer element T₁, andtransfer clock pulses φ₁ and φ₂ are alternately applied to the anodeelectrodes of the transfer elements and a write signal φ_(I) is appliedto the anode electrodes of the write light-emitting elements.

In the figure, R₁, R₂, R_(S) and R_(I) respectively show currentlimiting resistors.

The operation is briefly described. First, it is assumed that thevoltage of a transfer clock pulse φ₁ is an H level and the transferelement T₂ is on. At this time, the potential of the gate electrode G₂drops from 5 volts of V_(GK) to nearly zero volts. The influence of thispotential drop is transferred to the gate electrode G₃ by the diode D₂and sets its potential at about 1 volt (the forward threshold voltage ofthe diode D₂ (equal to the diffusion potential)). However, since thediode D₁ is inversely biased, the potential connection to the gateelectrode G₁ is not performed and the potential of the gate electrode G₁remains as 5 volts. Since the on-state voltage of a light emittingthyristor is approximated to a gate electrode potential+the diffusionpotential of a pn junction (about 1 volt), if the voltage of H level ofthe next transfer clock pulse φ₂ is set between about 2 volts (voltagenecessary for turning on the transfer element T₃) and about 4 volts(voltage necessary for turning on the transfer element T₅), only thetransfer element T₃ can be turned on and the other transfer elements canbe left as off state. Therefore, the on state is transferred by twotransfer clock pulses.

The start pulse φ_(S) is a pulse for starting such a transfer operationand the transfer element T₁ is turned on by setting the start pulseφ_(S) at H level (about 0 volt) and at the same time setting a transferclock pulse φ₂ at H level (about 2 to about 4 volts). Immediately afterthis, the start pulse φ_(S) is returned to H level.

Now, assuming that the transfer element T₂ is on, the potential of thegate electrode G₂ is about 0 volt. Therefore, if the voltage of a writesignal φ_(I) is not lower than the diffusion potential (about 1 volt) ofa pn junction, the light-emitting element L₂ can be made to come into alight emitting state.

On the other hand, the gate electrode G₁ is about 5 volts and the gateelectrode G₃ becomes about 1 volt. Accordingly, the write voltage of thelight-emitting element L₁ becomes about 6 volts and the write voltage ofthe light-emitting element L₃ becomes about 2 volts. From this, thevoltage of a write signal φ_(I) capable of writing into only thelight-emitting element L₂ comes to be in the range of 1 to 2 volts. Whenthe light-emitting element L₂ is turned on, namely, comes into a lightemitting state, the intensity of emitted light is determined by thequantity of electric current flowing through a write signal φ_(I) and animage can be written at an optional intensity. And in order to transfera light emitting state to the next light-emitting element, it isnecessary to lower the voltage of a write signal φ_(I) line to 0 voltonce and turn off the light-emitting element which is in a lightemitting state.

Industrial Applicability

The present invention enables a high-accuracy alignment withoutincreasing the number of components in assembling a resin lens array byproviding projections and depressions for alignment on resin lens platesforming the resin lens array and fitting the projections and thedepressions to each other.

And since the present invention can suppress the attenuation in quantityof light by providing an AR coat film on a resin lens plate forming aresin lens array, the present invention makes it possible to obtain anoptical write head having a large quantity of light.

And since the present invention has a structure in which the shape of afitting depression of a lens holder having a resin lens array placed init is longer than the resin lens array in the longitudinal direction soas to be capable of absorbing a strain caused by the difference incoefficient of thermal expansion between both the components, it ispossible to prevent a stress from occurring between both the componentseven when the ambient temperature changes.

And by providing a transparent cover over the optical output surface ofa resin lens array, the present invention can make a cleaning operationsimple thanks to being capable of flattening an optical output surfaceand can prevent the resin lens array from being scratched at the time ofperforming a cleaning operation thanks to being protected by thetransparent cover.

Further, since the present invention places a resin lens array in a lensholder having a flat supporting seat surface and always presses down theresin lens array against the lens holder by means of a metal retainer,the shape of the resin lens array is preserved.

And since the present invention makes it possible to easily mount aresin lens array in a lens holder by means of engaging portions providedon the lens holder, the process of production is made simple and thecost of production can be reduced.

Furthermore, since the present invention can absorb a strain caused bythe thermal expansion of a resin lens array by means of engagingportions provided on a lens holder and keep constant the distancebetween the resin lens array and a light-emitting element array chip, itmakes high-quality printing possible.

1. A resin lens array being formed by stacking one over another aplurality of resin lens plates each having spherical or asphericalmicrolenses regularly arranged at specific intervals on a flat plate,wherein: each of the plurality of resin lens plates has cone-shapedprojections formed at specific intervals outside the area in whichlenses are formed on one surface and depressions formed outside the areain which lenses are formed on the other surface, and the plurality ofresin lens plates are stacked so that said depressions are fitted tosaid projections.
 2. A resin lens array according to claim 1, wherein atleast one hole for making an adhesive agent escape when said projectionand said depression are made to adhere and fix to each other is formedin said depression to be fitted to said projection.
 3. A resin lensarray being formed by stacking one over another a plurality of resinlens plates each having spherical or aspherical microlenses regularlyarranged at specific intervals on a flat plate, wherein: each of theplurality of resin lens plates has projections formed outside the areain which lenses are formed on one surface and depressions formed outsidethe area in which lenses are formed on the other surface, theprojections being triangle-shaped in cross section and continuous orbeing triangle-shaped in cross section and successive at specificintervals, and the plurality of resin lens plates are stacked so thatsaid depressions are fitted to said projections.
 4. A resin lens arrayaccording to claim 3, wherein at least one groove for making an adhesiveagent escape when said projection and said depression are made to adhereand fix to each other is formed in said depression to be fitted to saidprojection.
 5. A resin lens array according to one of claims 1 to 4,wherein said resin lens plate has said microlenses arranged regularly atspecific intervals on both surfaces or one surface of it and said resinlens array is formed by stacking said resin lens plates one over anotherso as to have at least three or more lens forming surfaces.
 6. A resinlens array according to one of claims 1 to 4, wherein an antireflectioncoating for reducing the reflection of light is formed on the surface ofsaid resin lens plate.
 7. An optical write head collecting andprojecting light outputted from a light-emitting element array chiphaving light-emitting elements arranged in line on a photosensitivemember through a resin lens array formed by stacking one over another aplurality of resin lens plates each having spherical or asphericalmicrolenses regularly arranged at specific intervals on a flat plate,wherein: each of the plurality of resin lens plates has projectionsformed outside the area in which lenses are formed on one surface anddepressions formed outside the area in which lenses are formed on theother surface, and the plurality of resin lens plates are stacked sothat said depressions are fitted to said projections.
 8. An opticalwrite head according to claim 7, wherein said projections arecone-shaped projections provided at specific intervals.
 9. An opticalwrite head according to claim 8, wherein at least one hole for making anadhesive agent escape when said projection and said depression are madeto adhere and fix to each other is formed in said depression to befitted to said projection.
 10. An optical write head according to claim7, wherein said projections are projections being triangle-shaped incross section and continuous or successive at specific intervals.
 11. Anoptical write head according to claim 10, wherein at least one groovefor making an adhesive agent escape when said projection and saiddepression are made to adhere and fix to each other is formed in saiddepression to be fitted to said projection.
 12. An optical write headaccording to one of claims 7 to 11, wherein said resin lens array isformed by closely stacking one over another said resin lens plates eachhaving microlenses regularly arranged at specific intervals on bothsurfaces or one surface of it on a flat plate so as to have at leastthree or more lens forming surfaces.
 13. An optical write head accordingto one of claims 7 to 11, wherein an antireflection coating for reducingthe reflection of light is formed on the surface of said resin lensplate.
 14. An optical write head according to one of claims 7 to 11,wherein a transparent cover is arranged over the output surface of saidresin lens array and a metal retainer is attached to said transparentcover so that a pressing pressure is always applied to said resin lensarray.
 15. An optical write head according to one of claims 7 to 11,wherein said resin lens array is placed in a fitting depression formedin a supporting means for supporting said resin lens array and thelength of said fitting depression in the longitudinal direction islonger than said resin lens array in the longitudinal direction so as tobe capable of absorbing a strain caused by the difference in coefficientof thermal expansion in case that the strain caused by the difference incoefficient of thermal expansion occurs between said resin lens arrayand said supporting means.
 16. An optical write head according to one ofclaims 7 to 11, wherein said resin lens array is housed in an openingformed in a supporting means for supporting said resin lens array and issupported in said supporting means by engaging portions provided aroundsaid opening and having projections at the opening side.
 17. An opticalwrite head according to claim 16, wherein said engaging portions areformed at the longitudinal direction side and/or the perpendiculardirection side to the longitudinal direction of said opening.
 18. Anoptical write head according to claim 17, wherein said engaging portionsare formed at regular intervals or irregular intervals.
 19. An opticalwrite head according to claim 17, wherein said engaging portions areformed continuously or successively at regular intervals.
 20. An opticalwrite head according to claim 16, wherein said engaging portions aremolded into one body with said supporting means.
 21. An optical writehead according to claim 16, wherein said engaging portions are formedout of a material capable of absorbing the difference in displacementcaused by the difference in coefficient of thermal expansion betweensaid supporting means and said resin lens array.
 22. An optical writehead according to claim 16, wherein a sealing material is appliedbetween said supporting means and said resin lens array.
 23. An opticalwrite head collecting and projecting light outputted from alight-emitting element array chip having light-emitting elementsarranged in line on a photosensitive member through a resin lens arrayformed by stacking one over another a plurality of resin lens plateseach having spherical or aspherical microlenses regularly arranged atspecific intervals on a flat plate, wherein: said resin lens array ishoused in an opening formed in a supporting means for supporting saidresin lens array and is supported in said supporting means by engagingportions having projections at the opening side.
 24. An optical writehead according to claim 23, wherein said engaging portions are formed atthe longitudinal direction side and/or the perpendicular direction sideto the longitudinal direction of said opening.
 25. An optical write headaccording to claim 24, wherein said engaging portions are formed atregular intervals or irregular intervals.
 26. An optical write headaccording to claim 24, wherein said engaging portions are formedcontinuously or successively at regular intervals.
 27. An optical writehead according to one of claims 23 to 26, wherein said engaging portionsare molded into one body with said supporting means.
 28. An opticalwrite head according to one of claims 23 to 26, wherein said engagingportions are formed out of a material capable of absorbing thedifference in displacement caused by the difference in coefficient ofthermal expansion between said supporting means and said resin lensarray.
 29. An optical write head according to one of claims 23 to 26,wherein a sealing material is applied between said supporting means andsaid resin lens array.
 30. An optical write head according to one ofclaims 7 to 11 and 23 to 26, wherein said light-emitting element arraychip is a self-scanning light-emitting element array chip.